This invention relates to gas flow measurement and, more particularly, to the determination of the accuracy of a gas measurement instrument in a high pressure pipeline.
In co-pending application Ser. No. 734,937 filed on even date herewith, there is disclosed an arrangement for mounting a sonic nozzle in a gas pipeline for on-site calibration of a gas meter. When using that arrangement, since the gas velocity through the sonic nozzle is constant, knowing that velocity, the throat area of the nozzle, and the time during which gas flow is measured by the meter, a comparison may be made between the actual volume flow and the volume flow measured by the meter in order to determine the accuracy of the meter. The difficulty with this approach is in determining the velocity of the gas through the throat of the nozzle under sonic flow conditions. For a perfect gas, the velocity (v.sub.t) of the gas through the throat of a sonic nozzle under sonic conditions is EQU v.sub.t =.sqroot.kRT.sub.t ( 1)
Where:
k is the ratio of specific heats;
R is the gas composition dependent constant; and
T.sub.t is the throat temperature (.degree.R).
The basic problems with using this equation are that the throat temperature can't be measured and the quantities R and k are gas composition dependent.